I'm sorry to hear of your loss. It started out so good. Please don't beat yourself up. Yours is a VERY common mistake. If you read through some of the other posts, you'll see a lot of OOPS I didn't know about cycling my tank first. The LFS
is no help in this area, they want to sell you as much as you'll buy.
There isn't a lot you can do to save your dying Betta unless you know someone with a cycled tank who will take your Betta in while you cycle your tank.
If not and your Betta is suffering, you may need to....I don't like even saying it. But it is better than watching it suffer.
Once the tank is empty of fish you can start your fishless cycle. I'm not an expert on this, I've always cycled with fish. It's a lot of work with fish and they need to be very hardy fish.
If your doing a fishless cycle there are a couple of ways to start. One is start with fresh water and then add a capful of detergent free Ammonia. You can get it at your local hardware store.
If you know anyone with a cycled tank that has spare filter media that already has bacteria, it will speed up the process.
If not don't worry it will still work just a little slower.
If you don't want to add ammonia you can add food and let it disintegrate into ammonia. This can be messy, so I put a lot of fish pellet food into a small mesh bag and let it dissolve away. That way I don't have any fuzzy chunks to clean out of the tank. How you start is up to you>
Almost forgot, before you start anything you will need an API Freshwater Master test kit. Strips are a waste of money and they aren't accurate at all. The best price for the test kit is www.bigalsonline.com
, it's only $19.99 compared to Petsmarts $31.99.
There is also an article called; Beginner FAQ: Practical Water Chemistry.
What you need to know about water chemistry, and why.
It's not long but it will help and it will give you something to read while you wait for your test kit to arrive.
There are also numerous articles on how to cycle your tank. Which is a better idea than me typing it out step by step.
After you've read how it's done and you have your test kit, if you have some questions then please ask again. There are a lot of people here that like to help.This is a great place to get help, but the more you read and educate yourself, the better off you and your future fish will be.
If not me someone will be here for you, what ever you need to know.